Both selenium and arsenic are considered trace elements. Selenium much more so; most of the world's diet is selenium-poor. But selenium supplements are useful i

Both selenium and arsenic are considered trace elements. Selenium much more so; most of the world's diet is selenium-poor. But selenium supplements are useful in counteractive arsenic poisoning: https://www.ncbi.nlm.nih.gov/pubmed/23800687 https://www.ncbi.nlm.nih.gov/pubmed/23800687